Solar energy is becoming an increasingly feasible alternative to conventional fossil fuels for many applications. Efficient extraction of energy from the sun requires photovoltaic materials and devices capable of efficiently converting the energy contained in sunlight to electricity. The most common photovoltaic materials are semiconductors that convert sunlight to electricity by absorbing sunlight through a valence band to conduction band transition to produce the mobile charge carriers (electrons and holes) necessary to produce electrical current or voltage. Successful photovoltaic materials necessarily must possess high absorption efficiency over as much of the solar spectrum as possible and provide high mobility for the photo-generated charge carriers.
Most current photovoltaic devices are based on silicon. Silicon is a well-known electronic material and forms the basis for most modem semiconductor devices. As a photovoltaic material for solar energy devices, silicon provides high mobilities for charge carriers and readily permits integration of solar energy devices with other electronic devices. The absorption efficiency of silicon depends on its form. Crystalline silicon is an indirect bandgap semiconductor and has an intrinsically low absorption efficiency. Consequently, photovoltaic devices made from crystalline silicon are necessarily thick. Amorphous silicon, on the contrary, is a direct bandgap semiconductor and has a high absorption efficiency. The high absorption efficiency of amorphous silicon means that it can be used in thin film form in photovoltaic devices. From a processing point of view, amorphous silicon is preferable to crystalline silicon because it can readily be produced in a continuous manner over large areas in thin film form. Crystalline silicon, on the contrary, requires careful preparation in batch quantities through a slow, equilibrium growth process. As a result of the more convenient processing conditions and need for smaller quantities of material, amorphous silicon has emerged as the leading material for practical photovoltaic applications.
The economic viability of photovoltaic devices based on amorphous silicon depends critically on the ability to produce it in a high speed, continuous manufacturing process. The manufacturing process must permit the deposition of amorphous silicon over large area substrates in a wide variety of device configurations. One common device structure is the p-i-n structure. In this structure, three layers of amorphous silicon are present: an n-type layer, an intrinsic or undoped layer, and a p-type layer. Tandem device structures comprising a plurality of p-i-n structures are also common. The triple junction cell, for example, includes three stacked p-i-n structures, each of which is designed to absorb a different portion of the incident or solar spectrum. Optimal performance of layered amorphous silicon based photovoltaic devices requires stringent control over the thickness and chemical composition of the layers during manufacturing. Even small deviations from the intended specifications can significantly detract from device performance. Analogous considerations apply to related thin film photovoltaic materials such as germanium or alloys of amorphous silicon with germanium non-silicon based thin films, or even thicker crystalline or polycrystalline devices.
Consequently, a need exists for precise quality control during the high speed manufacturing of photovoltaic devices comprised of thin film layers. The most effective high-speed manufacturing processes are web based deposition processes. In these processes, deposition of layers of thin film photovoltaic materials to form a device occurs onto a moving web of a substrate material such as stainless steel. The web is fed from a source roll into a deposition chamber and is collected by a take-up roll after deposition of the desired combination of thin film layers. Web lengths of hundreds to many thousand feet can be used. Current quality control methods emphasize evaluating material properties and device performance after take-up has occurred. Typical post-deposition quality control methods involve removing the web or portions thereof from the take-up roll and depositing a conducting material such as ITO on the last deposited layer of a photovoltaic structure as a prerequisite to testing. The conducting material represents a physical electrical contact to the photovoltaic structure and permits measurement of relevant performance parameters such as open circuit voltages, short circuit currents, fill factor, shunt resistance and series resistance through a quality control device or method.
Although the post-processing, contacting quality control devices and methods of the prior art are capable of providing the relevant information needed to evaluate the performance and properties of photovoltaic devices, they suffer from an important disadvantage in that they offer slow information feedback rates. Web processing times are typically on the order of a day and it typically takes another few days to deposit the contacting conductive layer needed to perform quality control measurements. Consequently, quality control information is only available several days after culmination of the processing of the web. The long feedback time means that any problem that develops during processing is not known for days and that several days worth of material processed subsequent to the onset of the problem is potentially defective. Once a problem is detected, more production time is lost to relating the problem to process variables and to producing new material to verify that the problem has been properly remedied. The production of substantial amounts of defective product and the loss of valuable production time are costly and negatively impact the economic feasibility of photovoltaic and solar energy materials.
The present invention addresses the need for more immediate quality control assessment during the manufacture of photovoltaic devices. It is desirable to have a diagnostic quality control device or process for the manufacture of photovoltaic devices that provides a high information feedback rate and that permits on-line correction and optimization of the manufacturing process. The most common prior art quality control devices and processes are limited primarily by the need to form physical electrical contacts to the photovoltaic material or device being evaluated. Since this need cannot currently be met in a continuous manufacturing environment, prior art quality control is performed post-processing on a stationary web or portion thereof. The present invention describes a diagnostic device and process for quality control and evaluation that does not require the establishment of physical electrical contacts to the photovoltaic device being evaluated. The non-contacting diagnostic device and method of the present invention provides for the real-time evaluation of performance parameters of photovoltaic devices in a continuous manufacturing process.